Oil contamination in aircraft cabin air is undesirable. Typically, pressurized air provided to aircraft cabins is bled from an engine aboard the aircraft. However, the bled air can become contaminated with oil. Even a small amount of oil contamination is detectable by people in the aircraft cabin. One source of oil contamination is worn carbon seals used in aircraft engines. As the seals deteriorate, the risk of bleed air contamination increases as small amounts of oil may leak past the seal. In order to avoid contamination incidents, carbon seals are replaced often, and perhaps earlier than may actually be necessary. However, this is not a very cost effective way to optimize the use of carbon seals. Room for improvement exists.